Composite stones on a refractometer.

I wonder if anyone out there can help me.
I have a ring with what looks like a good coloured blue sapphire claw
set surrounded by three poor quality diamonds on each shoulder. It is
probably edwardian in age. From looking at the sapphire I am sure
there is something not right about it. On the refractometer I get the
usual sharp shadow edges for corundum, however at the top of the scale
around 1.5 to 1.6 there is a fairly faint patchy shadow. This shadow is
not there on other stones so it is not the equipment. Due to setting
limitations I havent turned up any obvious boundaries and I am
possitive that both crown and pavillion are natural sapphire (silk
visible). Could I be looking at a corundum corundum doublet. Also under
the polariscope a large patch of bright interference colour is visible,
would this indicate the boundary (un-natural cleavage if you
like)between the two sections. Just to confuse matters its the
strangest cut. From above the crown facets look like noral
cushion/round cut, but the pavillion facets are very shallow and also
look like crown facets. It like someone has cut two crowns of two
sapphires and stuck them back to back.
Has anyone seen this before or has any other tests I could do to come
up with an answer.

Thanks

Alastair Bickerstaff

I THINK some glues/oils that might be used to make a doublet would
fluoresce. ???

Carl


wrote:
I wonder if anyone out there can help me.
I have a ring with what looks like a good coloured blue sapphire claw
set surrounded by three poor quality diamonds on each shoulder. It is
probably edwardian in age. From looking at the sapphire I am sure
there is something not right about it. On the refractometer I get the
usual sharp shadow edges for corundum, however at the top of the scale
around 1.5 to 1.6 there is a fairly faint patchy shadow. This shadow is
not there on other stones so it is not the equipment. Due to setting
limitations I havent turned up any obvious boundaries and I am
possitive that both crown and pavillion are natural sapphire (silk
visible). Could I be looking at a corundum corundum doublet. Also under
the polariscope a large patch of bright interference colour is visible,
would this indicate the boundary (un-natural cleavage if you
like)between the two sections. Just to confuse matters its the
strangest cut. From above the crown facets look like noral
cushion/round cut, but the pavillion facets are very shallow and also
look like crown facets. It like someone has cut two crowns of two
sapphires and stuck them back to back.
Has anyone seen this before or has any other tests I could do to come
up with an answer.

Thanks

Alastair Bickerstaff



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On Sat, 11 Jun 2005 08:49:44 -0700, in wrote:

I wonder if anyone out there can help me.
I have a ring with what looks like a good coloured blue sapphire claw
set surrounded by three poor quality diamonds on each shoulder. It is
probably edwardian in age. From looking at the sapphire I am sure
there is something not right about it.

Trusting one's instincts in this sort of case is often a very good idea...

On the refractometer I get the
usual sharp shadow edges for corundum, however at the top of the scale
around 1.5 to 1.6 there is a fairly faint patchy shadow. This shadow is
not there on other stones so it is not the equipment.

Might be an inclusion, but also might be caused if the facet is not flat, so
you're getting a faint reading from a facet that's not properly in contact with,
and paralell, to the hemishere. long shot, but check that facet. prongs lifting
the stone very slightly above the hemisphere so you don't have perfect contact can
also do odd things.

Due to setting
limitations I havent turned up any obvious boundaries and I am
possitive that both crown and pavillion are natural sapphire (silk
visible).

Garnets can also have silk. Garnet/corundum doublets aren't common, but I've seen
them once or twice.

Could I be looking at a corundum corundum doublet.

Very possible. Best check is just very good examination with as high
magnification as you have available. This is the sort of place where a microscope
becomes invaluable. Seams in doublets can be hard to see if right at the girdle
plane, but with enough care, you usually can find them.

Also under
the polariscope a large patch of bright interference colour is visible,
would this indicate the boundary (un-natural cleavage if you
like)between the two sections.

Could also be twinning of two crystals. Or if a doublet, you may be seeing an
interference pattern generated by one half of the doublet?

Just to confuse matters its the
strangest cut. From above the crown facets look like noral
cushion/round cut, but the pavillion facets are very shallow and also
look like crown facets. It like someone has cut two crowns of two
sapphires and stuck them back to back.

An odd cut, but don't base an ID on the cut. Almost anything that can be cut, has
been cut, and it's hard to derive identification conclusions on cutting.

Has anyone seen this before or has any other tests I could do to come
up with an answer.

If you have some methelyene iodide (not refractometer fluid, just the plain
liquid) to use as an immersion liquid, doublets and other assembled stones are
MUCH easier to detect when immersed. Polariscope tests while immersed are also
useful. Barring Methylene Iodide, other higher refractive index liquids may also
be good enough. But the combination of immersion, polariscope, and microscope
(also while immersed, which can seem to require more than two hands at times),
should be able to give you your answer.

Peter

Thanks for the advise everyone,

The customer wants the stone un set so I will have a chance to really
get at it with the microscope and immersion cell, hopefully this will
help. Due to the strangly cut pavillion facets it is very misleading
when viewed through the table. There are what look like facet edges and
changes in lustre all over the place so finding a doublet boundary may
be difficult. We will see what the immersion cell has to say!

Alasdair

If you suspect a marriage of 2 different materials, specific gravity
could also be helpful.

sounds like fun!

Carl


wrote:

Thanks for the advise everyone,

The customer wants the stone un set so I will have a chance to really
get at it with the microscope and immersion cell, hopefully this will
help. Due to the strangly cut pavillion facets it is very misleading
when viewed through the table. There are what look like facet edges and
changes in lustre all over the place so finding a doublet boundary may
be difficult. We will see what the immersion cell has to say!

Alasdair



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